1. Field of the Invention
This invention relates to a technique of fabricating flat panel display devices, and more particularly to an apparatus and method for patterning an electro-luminescent display device for forming pixels into minute patterns in such an electro-luminescent display device.
2. Description of the Related Art
Recently, various flat panel display devices have been developed which are reduced in weight and bulk, thereby eliminating several disadvantages of a cathode ray tube (CRT). Such flat panel display devices include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and an electro-luminescent (EL) display device. Heightening the display quality of flat panel display devices and providing flat panel displays with a large-scale screen have been actively investigated. The PDP has been highlighted as a display device having the advantages of a light weight, a thin thickness and a small bulk as well as a large-scale screen owing to its simple structure and manufacturing process. However, the PDP has the drawbacks of a low emission efficiency, a low brightness and a high power consumption.
An active matrix LCD employing thin film transistors (TFT's) as switching devices is difficult to manufacture as a large-scale screen. An active matrix LCD exploits the efficiencies of semiconductor processing techniques, and has been largely used as a display device for notebook computers. Consequently, demand for large-scale screens has been insufficient to justify large-scale semiconductor processing. However, the LCD has large drawbacks in that it is difficult to provide a large screen area and power consumption is high due to a backlight unit. Also, the LCD has the undesirable characteristics of a large light loss and a narrow viewing angle due to a polarizing filter, a prism sheet and a diffuser, etc.
EL display devices may be classified into inorganic EL devices and organic EL devices, depending on the type of material used in a light-emitting layer. Such a device is “self-emitting,” emitting its own light. The EL display device has the great advantages of a rapid response speed, high emission efficiency, good brightness, and a large viewing angle.
In the organic EL display device as shown in FIG. 1, an anode electrode 31 composed of a transparent electrode pattern is provided on a glass substrate 2, and a hole injecting layer 32, a light-emitting layer 33 and an electron injecting layer 34 are sequentially disposed thereon. A cathode electrode 35 composed of a metal electrode is provided on the electron-injecting layer 34. When a driving voltage is applied to the anode electrode 31 and the cathode electrode 35, holes within the hole injecting layer 32 and electrons within the electron injecting layer 34 migrate toward the light-emitting layer 33 to excite a fluorescent material within the light-emitting layer 33. A picture or an image is displayed by the visible light generated from the light-emitting layer 33 in this manner. It is difficult to manufacture such an EL display device with a large-scale screen, because current mass-production techniques and processes are inadequate to repetitively manufacture a screen of more than 10 inches.
Studies of patterning pixels of the EL device have been made, but it is not yet conventionally possible to make a minute pattern and to make a repetitive manufacturing of red, green and blue pixels for a large-scale device. For example, an organic EL material cannot be patterned by wet etching because it is liable to be melted by a solvent or moisture. For this reason, the organic EL material cannot be patterned by photolithographic techniques which are advantageous for formation of minute patterns. A low-molecule organic EL material may be patterned using a method of independently forming each of red, green and blue materials using a minute-patterned shadow mask, but such a technique is limited by the accuracy with which shadow masks may be constructed. Such masks do not have a resolution beyond a certain level, and are difficult to accurately use over a large field due to a tension deviation, etc. of the shadow mask. A method of patterning pixels using an ink-jet injection head for a high-molecule or polymer organic EL material has been studied. However, it is difficult to form a pinhole-free thin film of less than 1000 Å thickness using such a method. A scheme of providing color filters on a white EL material, or of providing a color changing medium on a blue EL material, has been considered, but such a scheme causes a large light loss due to the color filters or the color changing medium.